1. Technical Field
The invention relates to a device for controlling the flow of plastics material, heated until flowable, through a housing, in particular a filtering device or a direction-control device for the plastics material, comprising a flow channel extending through the housing, and a plunger of circular cross-section longitudinally displaced in a bore of the housing transversely to the longitudinal direction of the flow channel and crossing the flow channel, wherein an unslotted scraper ring is mounted outside the housing on a portion of the plunger projecting from the housing, a stop connected to the housing being provided for the scraper ring so that the plunger is displaceable relative thereto.
2. Related Art
Various types of these devices are known. One example is a filtering device for plasticised thermoplastic material, in which the plunger carries a filter arranged in the flow channel when the device is in the operating position, the filter filtering out impurities from the plasticised plastics material. Another type is a device for changing the direction of flow of the plastics material. In this case, the flow of the plastics material through the housing is always controlled by the longitudinal displacement of the plunger. In the case of a filtering device, the aforementioned displacement of the plunger moves the filter into a screen-changing position or a backwashing position so that the filter can be replaced or cleaned (e.g. AT 395 825 B, EP 250 695 B). In the case of a direction-control device, the displacement of the plunger causes the plastics material introduced into the housing to leave the housing through a different outlet opening than during the normal operating state or, for example in the case of a gate valve, the plastics material is totally prevented from flowing through the housing.
In devices of the described type, plastics material unavoidably escapes from the housing during the longitudinal displacement of the plunger, even though the plunger is displaced in the housing with minimum clearance to ensure as little leakage as possible. The plastics material forcibly drawn out of the interior of the housing during the movement of the plunger is decomposed by heat and the effect of atmospheric oxygen. The resulting carbonized mass forms a hard layer which surrounds the plunger and which has substantially lost its plastics character. The purpose of the scraper ring is to scrape this hard layer of leaked material off the plunger during its longitudinal displacement relative to the housing. However, tests have shown that the following difficulties arise here:
The portion of the plunger projecting from the housing is not heated and is therefore subject to a temperature decrease of approximately 100xc2x0 C. over its axial, non-heated length. This means that the diameter of the plunger is smaller at its end projecting, from the housing than in the remaining portion by some tenths of a millimeter, depending on the size of this diameter, owing to the reduced thermal expansion in accordance with its lower temperature. Therefore, when the plunger moves into the heated housing, the burnt-on layer of material is scraped off by the housing edge or the scraper ring only until it corresponds to the bore diameter of the housing or the inner diameter of the scraper ring. Consequently, a layer of this material remains at the colder end of the plunger and is of a thickness conforming to the smaller diameter attributable to the reduced thermal expansion of this end of the plunger. During the dwell time of the plunger in the position in which it is retracted into the housing, e.g. the filtering position in the case of a filtering device or the normal position of the direction-control device or the like, this plunger portion heats up to the housing temperature and therefore expands accordingly. The coating of carbonized plastic on the plunger is densified as a result. This uncontrollably impedes the movement of the plunger back out again, possibly even jamming the plunger and with it the entire device.
The object of the invention is to overcome these difficulties and improve a device of the initially described type so that its operation is more reliable.
The invention achieves this object by the inner diameter of the scraper ring being smaller than the diameter of the bore of the housing and also smaller than the diameter of the end of the plunger portion remote from the housing and projecting therefrom in the coldest operating state of that plunger portion so that the scraper ring is always pretensioned on the latter. This close fit of the scraper ring on the plunger means that the scraper ring is always pretensioned on the plunger, irrespective of the temperature-induced expansions or contractions which the plunger undergoes. Naturally, this means that corresponding tensile stresses occur in the scraper ring, but selecting the material for the scraper ring, in particular its heat resistance values, so that the tensile stresses occurring are absorbed without causing the scraper ring to fracture does not give rise to any difficulties because the temperatures occurring are at least substantially known.
According to the invention, the inner diameter of the scraper ring is preferably smaller than the diameter of the bore of the housing by 0.12% to 0.15%. Experiments have shown this range to be suitable for most applications.
The amount of pretension with which the scraper ring is mounted on the plunger portion is to be selected in accordance with the prevailing conditions. Generally, within the framework of the invention, the arrangement is such that the scraper ring is mounted on the plunger with an average contact pressure of 1000-2000 N/cm2.
According to a further development of the invention, a clearance of 0 mm to 4 mm, measured in the axial direction of the plunger and bounded by the stop, is provided between the scraper ring and the housing. The zero value, i.e. no clearance, applies to devices used for low-viscosity plastics, e.g. PET premonomers or waxes or the like. With these plastics, there is also the danger of material leaking out when the plunger is extended and there is high mass pressure in the housing. In this case, the scraper ring without axial clearance acts like a gland. For high-viscosity plastics, however, the aforementioned amount of clearance between the scraper ring and the housing is possible and, in the region of this axial clearance, leaked material can issue freely when the plunger is extended.
The subject of the invention is shown schematically in the drawings by means of embodiments.